This study aims to investigate the effects of specimen thickness and fiber length on the tensile and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). To this end, a uniaxial tensile test was conducted with three specimen thicknesses (30, 50, and 100 mm) and two fiber lengths (13 and 20 mm), and the fiber orientation, dispersion and specimen void were quantitatively evaluated based on image recognition. The test results indicated that fiber orientation was improved with the decreased specimen thickness and increased fiber length. Meanwhile, the initial cracking and peak stress, capacity to limit cracking were enhanced with the decreased specimen thickness. A modified prediction model considering the wall effect, flattening and squeezing effect was developed to predict the probability density function p(θ) of fiber orientation angle. Additionally, the uniformity factor μ2 was introduced to predict crack number, and the relationship between the μ2 and parameter ψ = (Vf × lf/df)/t was determined. Furthermore, a model was developed to convert the main crack width into uniaxial tensile strain. All models and relationships were validated using test data. A micromechanical model that considered the predicted p(θ) and conversion model was established to predict the uniaxial tensile response of UHPFRC, which was also successfully validated using test data.

中文翻译：

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试件厚度和纤维长度对 UHPFRC 拉伸和开裂行为的影响：单轴拉伸试验和微观力学建模


本研究旨在研究试件厚度和纤维长度对超高性能纤维增强混凝土 （UHPFRC） 的拉伸和开裂行为的影响。为此，对三种试样厚度（30、50 和 100 mm）和两种纤维长度（13 和 20 mm）进行了单轴拉伸试验，并基于图像识别定量评价了纤维取向、色散和试样空隙。试验结果表明，随着试样厚度的减小和纤维长度的增加，纤维取向得到改善。同时，初始开裂和峰值应力、限制开裂的能力随着试样厚度的减小而增强。建立了一种考虑壁面效应、压平和挤压效应的改进预测模型，用于预测纤维取向角的概率密度函数 p（θ）。此外，引入均匀性因子 μ2 来预测裂纹数量，并确定了 μ2 与参数 ψ = （Vf × lf/df）/t 之间的关系。此外，还开发了一种模型，将主要裂纹宽度转换为单轴拉伸应变。所有模型和关系都使用测试数据进行了验证。建立了一个考虑预测 p（θ） 和转换模型的微观力学模型来预测 UHPFRC 的单轴拉伸响应，该模型也使用测试数据成功验证。